Plasma processing apparatus with reduced parasitic capacity and loss in RF power

ABSTRACT

A plasma processing apparatus comprises a grounded housing, a thin RF plate electrode, an opposite electrode facing the RF plate electrode, and a RF power source for applying a radio frequency to either the RF plate electrode or the opposite electrode to produce plasma between the two electrodes. If the radio frequency applied to the electrode is f (MHz), the parasitic capacity C (pF) between the grounded portion of the housing and a conductive portion through which the radio frequency propagates is less than 1210*f −0.9 . The thickness of the RF plate electrode is 1 mm to 6 mm, and it is supported by a heat sink. The heat sink has a coolant passage in the proximity to the RF plate electrode. The heat sink also has a groove or a cavity in addition to the coolant passage, thereby reducing the value of the dielectric constant of the heat sink as a whole.

The present patent application claims the benefit of earlier JapanesePatent Application No. 2000-195165 filed Jun. 28, 2000, the disclosureof which is herein incorporated entirely by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma processing apparatus forproducing plasma under application of a radio frequency and for carryingout etching or CVD processes.

2. Description of the Related Art

A parallel plate type plasma etching apparatus is generally used insemiconductor manufacturing processes. In the conventional plasmaetching apparatus, a radio frequency of about 13.56 MHz is applied tothe cathode electrode to excite plasma. However, in order to keep upwith increasingly strict design rules, and in order to respond to ademand for improvements in productivity, techniques of applying a higherrange of radio frequency, e.g., the frequency band of VHF to UHF havebeen studied. The proposal of raising the radio frequency applied to theelectrode also responds to a demand for an increase in wafer size, inwhich more intricate patterns are to be formed.

However, as the frequency applied to the electrode becomes higher, theloss of RF power also increases. If the loss of RF power increases, theelectron density of plasma produced between the parallel plateelectrodes decreases, lowering the etching rate. Consequently, a wafercannot be precisely etched into a designed shape and pattern.

FIG. 1 illustrates a conventional plasma processing apparatus. Theconventional apparatus comprises an RF electrode 4, to which an RF isapplied, and a metallic DC plate 2, to which a direct-current voltage isapplied, a ceramic or resin insulator 3 a, and an opposite electrode 5facing the RF electrode. The conventional RF electrode 4 is maderelatively thick because the RF electrode itself has a wafer supportfunction. The RF electrode 4, the DC plate 2, and the insulator 3 a and3 b constitute a wafer hold structure.

As illustrated in FIG. 2, pusher pins 12 extend penetrating the RFelectrode 4, the DC plate 2 and the insulator 3 b. The pusher pins 12are used to place a wafer 1 onto the RF electrode 4. When the wafer 1 istransported into the housing 7, the pusher pins 12 elevate and projectabove the RF electrode 4 to receive the wafer 1. Then, the pusher pins12 are lowered to place the wafer 1 onto the insulator 3 b.

The wafer 1 is securely held on the RF plate 4 by an electrostatic chuckconsisting of the insulator 3 b and the DC plate 2. A positive voltageof 1000V to 3000V is applied to the DC plate 2 from the DC power source11. In this situation, if a radio frequency is applied to the RFelectrode 4 to produce plasma, the wafer 1 is charged up negatively andattracted to the DC plate 2 that is at a positive voltage. A low passfilter 13 prevents the RF power, which is applied to the RF electrode 4and transferred to the DC plate 2 via the parasitic capacity between theRF electrode 4 and the DC plate 2, from flowing into the DC power source11.

In the conventional plasma processing apparatus shown in FIGS. 1 and 2,as the radio frequency applied to the electrode is raised, loss due tothe inductance and the parasitic capacity of the hot lines becomeslarge. This means that the loss of the RF power also increases.

The most significant example of loss due to the raised radio frequencyis the growth in parasitic capacity relative to the capacity of producedplasma. To be more precise, as the radio frequency becomes higher, theplasma density increases, as illustrated in FIG. 4B. On the contrary,the plasma capacity itself abruptly decreases as the frequencyincreases, as shown in FIG. 4A. For this reason, the parasitic capacityexisting between the RF supply line through which the radio frequencypropagates and the grounded portion of the housing 7 becomes almostequal to the plasma capacity (that is, the parasitic capacity increasesrelatively). This means that, apart from that used for producing andmaintaining plasma, the RF power supplied to the apparatus is wasted.

Another problem in the conventional plasma processing apparatus is theparasitic capacity existing between the RF mount electrode 4 and thepusher pins 12. As is illustrated in FIG. 2, the conventional RF mountelectrode 4 is made relatively thick because it is designed to functionas both an electrode and a wafer mount stage. The pusher pins 12 alwaysface the metallic electrode 4 even after it retreats inside the RF mountelectrode 4, producing parasitic capacity between the pins and electrode4. This parasitic capacity causes a loss in RF power.

Such a loss becomes particularly marked if the applied radio frequencyis 60 MHz or higher. Accordingly, the reduction of parasitic capacity isone of the most serious problems to be solved. In order to reduce theparasitic capacity, the entire apparatus, including the heat sinkstructure and pusher pin arrangement, must be configured optimally.

Still another problem in the conventional apparatus is the loss of RFpower from the electrostatic chuck. As has been mentioned above, theradio frequency applied to the RF mount electrode flows into the lowpass filter 13 via the DC transmission line, and is consumed in thisfilter. This occurs because the low pass filter 13 consists oflumped-constant reactance elements, and has large parasitic capacity. Asthe radio frequency becomes higher, loss or the consumption in the lowpass filter 13 increases. The radio frequency flowing into the low passfilter 13 may cause the break down or burning of the low pass filter,and may damage the DC power source 11.

On the other hand, it is desirable to reduce the volume of the housing 7as much as possible in order to produce plasma efficiently, whilereducing the quantities of precursor gases introduced into theapparatus. To reduce the volume of the apparatus, it has been proposedto use the heat sink (or insulator) that supports the RF electrode and awafer as a vacuum chuck itself. However, the interface between themetallic housing and the ceramic sink is located at the boundary betweenthe vacuum and the atmosphere. Consequently, the heat sink becomesbrittle and is likely to break.

SUMMARY OF THE INVENTION

Therefore, to overcome the problems in the prior art technique, a plasmaprocessing apparatus that can reduce a loss of RF power even if a radiofrequency of 60 MHz or higher is applied to the electrode is provided inone aspect of the invention. In this apparatus, the plasma capacity isincreased relative to the parasitic capacity by reducing the parasiticcapacity of the apparatus as a whole. This plasma processing apparatuscomprises a grounded housing, a thin RF plate electrode placed in thehousing, an opposite electrode facing the RF plate electrode, and an RFpower source for applying a radio frequency to either the RF plateelectrode or the opposite electrode. By applying a radio frequency toeither electrode, plasma is produced between the RF plate electrode andthe opposite electrode. If the radio frequency applied to the electrodeis f (MHz), the parasitic capacity C (pF) between the grounded portionof the housing and a conductive portion through which the radiofrequency propagates is less than 1210*f^(−0.9).

In another aspect of the invention, a plasma processing apparatuscomprises a grounded housing, an RF plate electrode placed in thehousing, an opposite electrode facing the RF plate electrode, and firstand second radio-frequency power sources. The first and secondradio-frequency power sources apply different values of radiofrequencies to either the RF plate electrode or the opposite electrode.One of the radio frequencies applied to the electrode is 60 MHz orhigher. If this radio frequency is f (MHz), the parasitic capacity C(pF) between the grounded portion of the housing and a conductiveportion (or hot lines) is also less than 1210*f^(−0.9).

In still another aspect of the invention, a plasma processing apparatushaving an improved pusher pin structure for reducing the parasiticcapacity is provided. In this apparatus, the parasitic capacity betweenthe RF plate electrode, to which a radio frequency is applied, andpusher pins is substantially eliminated, thereby greatly reducing theloss of RF power. This plasma processing apparatus comprises a groundedhousing, a wafer mount electrode having at least two holes passingthrough it, an opposite electrode facing the wafer mount electrode, anRF power source, and pusher pins inserted in the holes. The wafer mountelectrode includes an RF plate electrode having a thickness of 6 mm orless and an insulator for supporting the RF plate electrode. The pusherpins are movable between a first position, at which the pusher pinsproject out of the wafer mount electrode to receive a wafer, and asecond position, at which the pusher pins retreat below the RF plateelectrode during the generation of plasma.

In yet another aspect of the invention, a plasma processing apparatusthat can reduce the loss of RF power, eliminate adverse influence to theDC power source, and produce plasma at a high density, is provided. Thisplasma processing apparatus comprises a wafer mount electrode, anopposite electrode facing the wafer mount electrode, a DC power source,and RF power source, and a radio-frequency trap positioned between thewafer mount electrode and the DC power source. The DC power sourcesupplies a direct-current voltage to hold a wafer on the wafer mountelectrode in an electrostatic manner. The RF power source applies aradio frequency to either the wafer mount electrode or the oppositeelectrode to generate plasma between the two electrodes. Theradio-frequency trap has an electrical length of (2n+1)/4 wavelength ofthe applied radio frequency, where n is 0 or a natural number.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages will be apparent from the followingdetailed description of the invention in conjunction with the attacheddrawings, in which:

FIG. 1 illustrates a conventional plasma processing apparatus;

FIG. 2 illustrates the positional relationship between the wafer mountelectrode and the pusher pins used in the conventional plasma processingapparatus;

FIG. 3 illustrates a plasma processing apparatus according to the firstembodiment of the invention;

FIG. 4A is a graph showing the plasma capacitance as a function offrequency applied to the plasma processing apparatus, and FIG. 4B is agraph showing the theoretical electron density and the actual electrondensity reflecting decrease in plasma capacitance in the higherfrequency range;

FIG. 5 illustrates a modification of the plasma processing apparatusshown in FIG. 3; and

FIG. 6 illustrates another modification of the plasma processingapparatus shown in FIG. 3;

FIG. 7 illustrates the positional relationship between the wafer mountelectrode and pusher pins according to the second embodiment of theinvention;

FIG. 8A is a plan view of the wafer placed on the pusher pins using afolk, FIG. 8B is a cross-sectional view taken along the C—C line of FIG.8A, and FIG. 8C is a cross-sectional view of the wafer mounted on thewafer mount electrode with the pusher pins retreated inside theinsulating plate; and

FIG. 9 illustrates a plasma processing apparatus according to the thirdembodiment of the invention, which uses an RF trap with an electricallength of λ/4.

DESCRIPTION OF THE EMBODIMENTS

(First Embodiment)

FIG. 3 illustrates a plasma processing apparatus 100 according to thefirst embodiment of the invention. The plasma processing apparatus 100has housing 107, a thin RF plate electrode 104 placed in the housing,and an opposite electrode 105 facing the RF plate electrode 104. Theapparatus also has a heat sink 103 holding the RF plate electrode 104,and an RF power source 109 for applying a radio frequency to at leastone of the RF plate electrode 104 and the opposite electrode to produceplasma between these two electrodes. A matching box 131 is insertedbetween the RF power source 109 and the load to cancel the reactancecomponent of the load and correct the impedance.

The RF plate electrode has a thickness of 6 mm or less, and morepreferably, 1 mm to 3 mm, which is relatively thin as compared with theconventional RF electrode. The thin RF plate electrode 104 is held onthe heat sink 103, which regulates the temperature of a wafer that is tobe processed. The heat sink 103 is, for example, a cylinder made ofceramics, and has a coolant passage 119. The coolant passage 119 islocated just under the RF plate electrode 104 to regulate thetemperature of a wafer 101 during plasma processing. To carry out plasmaprocessing with high precision, the wafer temperature must be keptuniform.

In addition to the coolant passage 119, a groove 120 is formed in theheat sink 103. By forming the coolant passage 119 and the groove 120,the dielectric constant of the heat sink 103 can be reduced as a whole,and consequently, the parasitic capacity existing in the housing 107decreases. The coolant passage 119 and the groove 120 are annular orcurved.

The heat sink 103 is secured at the bottom edge on the shoulder 125 ofthe housing 107. The diameter of the housing 107 becomes smaller at theshoulder 125. By mounting the heat sink 103 on the shoulder 125 of thehousing 107, a vacuum chamber is formed in the upper part of the housing107. Thus, the heat sink 103 functions as a vacuum chuck. From itsbottom face, the heat sink 103 is in contact with the atmosphere.

In general, if the interface between the grounded housing 107, which ismade of a conductive material, and the heat sink 103, which is made of aceramic material, is located at the boundary between the vacuum and theatmosphere, the heat sink is likely to break or crack. To prevent suchdamage, a shock absorber 117 is inserted between the heat sink 103 andthe housing 107. The shock absorber 117 is made of a soft and malleableinsulator, such as Teflon. By inserting a shock absorber between thehousing (i.e., the conductor) and the heat sink (i.e., the insulator,such as ceramics), the heat sink is protected from breakage even if itis located at the boundary between the vacuum and the atmosphere.

As a feature of the plasma processing apparatus, if the radio frequencyapplied to the RF plate electrode 104 is f (MHz), the parasitic capacityC (pF) existing in the housing is less than 1210*f^(−0.9). By limitingthe parasitic capacity to this value, the plasma capacity is increasedrelative to the parasitic capacity, and as a result, the plasma densitycan be increased.

This concept will be explained with reference to FIGS. 4A and 4B. FIG.4A is a graph of the plasma capacity (pF) as a function of the radiofrequency f (MHz) applied to the RF plate electrode 104 shown in FIG. 3.As is clearly shown in the graph, as the radio frequency applied to theelectrode is raised, the plasma capacity abruptly decreases. Therelation between the plasma capacity and the applied frequency is

C(pF) =1209.9×f ^(−0.9016).  (1)

It is generally convinced that as the applied radio frequency becomeshigher, the electron density in the produced plasma increases. Thegeneral understanding was that electron density is in proportion to thesquare of the applied frequency, and therefore, it was believed thatplasma processing could be carried out more efficiently by theapplication of higher RF power.

However, in reality, the produced plasma capacity decreases at higherfrequency, as is shown in FIG. 4A. If the parasitic capacity of thecircuit (that is, the capacity generated between the ground and theconductive passes through which the radio frequency propagates) is, forexample, about 10 pF as indicated by the dashed line C in FIG. 4A, then,the produced plasma capacity becomes almost equal to the parasiticcapacity if the applied radio frequency is raised up to 200 MHz. Thismeans that the applied radio frequency is wasted on irrelevant activityother than producing plasma. If the radio frequency is further raised,the produced plasma capacity falls below the dashed line C, which meansthat the parasitic capacity is greater than the produced plasmacapacity.

FIG. 4B is a graph of electron density as a function of appliedfrequency in a conventional parallel plate plasma etching apparatusdesigned for 13.56 MHz. The curved line L1 represents the theoreticalvalue of the electron density, which is in proportion to square ofapplied frequency (f²). The line L2 represents the actual electrondensity taking the decrease of produced plasma capacity into account. Ashas been explained in conjunction with FIG. 4A, the plasma capacitydecreases in proportion to f^(−0.9). Accordingly, the plasma density(i.e., the electron density) increases in proportion to power of 1.1 ofapplied frequency.

In order to guarantee that the increase in electron density is at leastin proportion to f^(1.1), the apparatus must be designed so that theparasitic capacity C is kept under 1210×f^(−0.9016) (pF) even if theapplied radio frequency is raised.

The parasitic capacity exists between the hot lines through which theradio frequency propagates and the grounded portion. The hot linesinclude, for example, the conductive extending from the RF power source109 to the RF plate electrode 104, and the RF plate electrode itself. Toreduce the parasitic capacity, the following measures are taken in theplasma processing apparatus of the first embodiment:

(i) selecting a coolant that has as low a dielectric constant aspossible (or at least lower than the dielectric constant of the heatsink 103);

(ii) forming a groove or a capacity in the heat sink 103; and

(iii) setting the dimensions of the elements of the apparatus optimally.

Item (iii) relates to the gap “a” between the opposite electrode 105 andthe wafer surface, the space “b” between the edge of the RF plateelectrode 104 and the housing 107, the distance “c” from the shoulder125 to the RF plate electrode 104, and spaces “e” and “f” from theconductive lines to the housing 107 being set optimally.

For example, a 300 mm wafer is processed in the apparatus, and the gap“a” between the opposite electrode 105 and the surface of the wafer 101is set to 20 mm. Under this condition, it is desirable to have a largespace “b” between the edge of the RF plate electrode 104 and the housing107 in order to reduce the parasitic capacity produced in this portion.However, the space “b” cannot be increased too freely because theapparatus would become huge. Therefore, the space “b” is set to abouttwice of “a”, that is, about 40 mm. The distance “c” from the bottom ofthe heat sink 103 to the RF plate electrode 104 is set to b* ∈ r (where∈ r is the relative dielectric constant of the heat sink 103).

The parasitic capacity C between the RF plate electrode 104 and theshoulder 125 of the housing 107, at which the heat sink 103 is held, isgiven by ∈ S/c (where ∈ is the dielectric constant of the heat sink 103and S is the contact area between the bottom of the heat sink 103 andthe shoulder 125 of the housing 107). If the distance “c” is set toosmall, the parasitic capacity becomes large. If the distance “c” is setlarge, the apparatus become large. For this reason, “c” is set equal tob* ∈ _(r) in order to guarantee the mechanical strength of the heat sink103 even allowing for the inclusion of the coolant passage 119 and thegroove 120, while preventing the apparatus from becoming too large.

If the heat sink 103 is made of quartz, the relative dielectric constant∈ r is about 6. If the heat sink 103 has no coolant passages 119 orgrooves 120, the distance “c” is 240 mm based on the calculation of b* ∈r. Under these conditions, the dielectric constant of the entirety ofthe heat sink 103 can be reduced by:

(i) Making the volume of the groove 120 as large as possible within arange not adversely affecting the mechanical strength of the heat sink103;

(ii) Using a coolant that has as low a dielectric constant as possible;and

(iii) Combining (i) and (ii).

Concerning item (i), the relative dielectric constant in the groove isabout 1, and therefore, as the groove becomes large, the dielectricconstant of the heat sink 103 as a whole becomes smaller. Concerningitem (ii), an example of the coolant is fluorine-containing inertliquid, such as Fluorinert (manufactured and sold by Sumitomo 3M Co.Ltd.) whose dielectric constant is about 2.5. The coolant passage islocated in proximity to the wafer in order to regulate the temperatureof the wafer 101.

It is also necessary to make the contact area S between the bottom ofthe heat sink 103 and the shoulder 125 of the housing 107 small in orderto reduce the parasitic capacity. However, if the contact area S isfixed small, the interface between the metallic housing and the brittleheat sink will be located very close to the boundary between the vacuumand the atmosphere. This causes the heat sink 103 to be easilybreakable. To overcome this problem, a shock absorber 117 is insertedbetween the bottom of the heat sink 103 and the shoulder of the housing107. In this arrangement, the parasitic capacity can be reduced withoutdamaging the heat sink 103. Consequently, plasma can be produceduniformly above the wafer at a high density, while minimalizing the lossof RF power.

FIG. 5 illustrates a modification of the plasma processing apparatusshown in FIG. 3. In the example of FIG. 5, two RF power sources areused, and two different radio frequencies are applied to the electrode.Matching boxes 231 and 232 are also provided in parallel. The first RFpower source 209 supplies a radio frequency of about 60 MHz to 100 MHzto generate plasma, and the second RF power source 210 supplies a radiofrequency of about 1 MHz to adjust the ion energy. By regulating thevoltages of these power sources, the plasma density and the ion energycan be balanced. To be more precise, the second RF power source 210 isregulated to control the etching rate and the pattern formation. Byreducing the ion energy, the plasma processing apparatus 200 can be usedfor CVD, for example, filling VIA holes.

A coolant passage 219 is formed in the heat sink 203 in proximity to thewafer 201, and a groove 220 is formed so as to be as large as possiblewithin a range that does not adversely affect the mechanical strength ofthe heat sink 203. The dimensions a, b, c, e, and f are selectedoptimally, as in the example shown in FIG. 3. The contact area (or theinterface) between the heat sink 203 and the housing 207 is fixed small,and a shock absorber 217 is inserted in this portion to protect the heatsink 203. With this arrangement, both the ion energy and the plasmadensity are controlled simultaneously, while reducing the parasiticcapacity efficiently, and highly precise plasma processing can becarried out.

FIG. 6 illustrates another modification of the plasma processingapparatus shown in FIG. 3. In this example, a cavity 330 is formed inaddition to the coolant passage 319, and the matching box 331 and afilter are accommodated in the cavity 330. The cavity 330 contributes toreducing the dielectric constant of the heat sink 303 as a whole, andconsequently, the parasitic capacity of the apparatus is reduced. Inaddition, the cavity 330 allows the apparatus to be made compact.

The heat sink 303 is supported on the on a part of the housing 307 witha cushion 317 inserted between the heat sink 303 and the housing 307.Again, the heat sink 303 itself functions as a vacuum chuck to define asealed space for producing plasma, while cracks or breakage of the heatsink 303 that is located at the boundary between the vacuum and theatmosphere is prevented.

<Second Embodiment>

FIG. 7 illustrates a wafer mount electrode 500 and pusher pinspenetrating the wafer mount electrode. As has been explained above, ifthe plasma processing is carried out at a higher range of radiofrequency, the parasitic capacity of the apparatus has to be reduced asmuch as possible in order to minimalize the loss of RF power. Thecapacitive coupling between the electrode and the pusher pins is one ofthe more significant factors regarding the parasitic capacity in theconventional plasma processing apparatus.

In the prior art, as illustrated in FIG. 2, the pusher pins 12 areelevated in the through-holes 15 to the position B when mounting to orremoving a wafer from the wafer mount electrode. During the generationof plasma, the pusher pins 12 are retracted to the position A inside theRF electrode 4 in the conventional apparatus. The loss of the RF powerdue to the capacitive coupling between the pusher pins 12 and the RFelectrode 4 is significant, and cannot be neglected. Because the pusherpins 12 play an important role in discharging residual charges, inaddition to receiving the wafer, the pins 12 cannot be replaced withinsulating pins. For this reason, the parasitic capacity of the pusherpins 12 that is capacitively coupled with the RF electrode 4 has beenthe main cause of the loss of the RF power.

Increasing the gap d between the pusher pin 12 and the RF electrode 4can reduce the capacitive coupling between the pusher pin and theelectrode. However, the distance from the inner face of the through-hole15 to the pusher pin 12 has to be kept to 0.8 mm or less to preventabnormal discharge due to plasma entering into the gap. This requirementprevents the reduction of the parasitic capacity C=∈ S/d, where S is thesurface area of the pusher pin 12 that faces the inner face of thethrough-hole 15. The area S is defined by the diameter and the height Lof the pusher pin 12.

The conventional RF electrode has a thickness of about 15 mm. Meanwhile,it is desirable that the vertical stroke of the pusher pin is set small,preferably, at 8 mm or less. Accordingly, it was difficult for theconventional structure to reduce the parasitic capacity between thepusher pin and the electrode.

To overcome this problem, the wafer mount electrode 500 of the secondembodiment is comprised of a thin RF plate electrode 504, and aninsulating plate 506 to reinforce the RF plate electrode 504, asillustrated in FIG. 7. The thickness of the RF plate electrode 504 is 6mm or less, and 3 mm of thickness is selected in the preferredembodiment. This arrangement efficiently prevents the pusher pin 502from being capacitively coupled with the wafer mount electrode 500during the generation of plasma. This wafer mount electrode can besuitably used in the plasma processing apparatuses shown in FIGS. 3, 5and 6.

The insulating plate 506 is bonded to the RF plate electrode 504. Thewafer mount side of the RF plate electrode 504 is covered with aninsulator 503, and a DC plate 502 is placed inside the insulator 503 toprovide an electrostatic chuck. The insulator 503 is made of the samematerial as the heat sink used in the plasma processing apparatusesshown in FIGS. 3, 5 and 6. The insulating plate 506 supporting the RFplate electrode 504 may be of the same material as the insulator 503, oralternatively, of a different material from the insulator 503.

The thickness of the RF plate electrode 504 of the wafer mount electrode500 is greatly reduced, as compared with the metallic portion of theconventional wafer mount electrode. Two or more through-holes 505 areformed in the wafer mount electrode 500, and pusher pins 502 made of aconductor or semiconductor are inserted in the through-holes 505. Thepusher pin 502 is driven by a driving mechanism (not shown) between theupper position B and the lower position A indicated by the dashed linesin FIG. 7. When mounting to and removing a wafer from the wafer mountelectrode 500, the pusher pins 502 project out from the wafer mountelectrode 500 up to position B. The pusher pins 502 are retracted toposition A, and are positioned below the RF plate electrode 504 duringthe generation of plasma. In position A, the pusher pins 502 face theinsulating plate.

FIG. 8 illustrates the vertical displacement of the pusher pins 502 moreclearly. A wafer 601 is placed on a fork 606 and transported into theplasma processing apparatus shown in FIG. 3, 5 or 6, as illustrated inFIG. 8A. Then, the wafer 601 is positioned over the wafer mountelectrode 500. At this time, the pusher pins 502 project from thethrough-holes 505 and receive the wafer 601, as illustrated in FIG. 8B.Upon placing the wafer 601 onto the pusher pins 502, the fork 606 leavesthe chamber (not shown).

Then, the pusher pins 502 are retracted into the through-hole and remainbelow the RF plate electrode 504, as illustrated in FIG. 8C. The wafer601 is held on the wafer mount electrode 500 in an electrostatic manner.Then, a radio frequency is applied to the RF plate electrode 504 tostart plasma processing. Since the pusher pins 502 are positioned belowthe RF plate electrode 504 and face the insulating plate 506, capacitivecoupling between the pusher pin 502 and the RF plate electrode can beavoided.

In the second embodiment, the stroke of the pusher pins 502 is only 6 mmbecause the thickness of the RF plate electrode is reduced to 3 mm. Thismeans that the pusher pin 502 is distanced from the RF plate electrode504 by about 3 mm. Consequently, the value “d” increases substantially,and the parasitic capacity between the pusher pin 502 and the RF plateelectrode 504 is reduced greatly.

Preferably, the stroke of the pusher pin 502 is 8 mm or smaller. If thestroke is set to the maximum (i.e., 8 mm), the pusher pin 502 is furtheraway from the RF plate electrode 504, and the parasitic capacity isfurther reduced. In any cases, it is preferable that the stroke of thepusher pins between the first and second positions is twice or more ofthe thickness of the RF plate electrode. Because a thin RF plateelectrode 504 is used, the stroke of the pusher pin 502 can be reduced.At the same time, the pusher pin 502 retreats completely into theinsulating plate 506 below the RF plate electrode 504, and the parasiticcapacity between the pusher pins 502 and the electrode 504 can beeliminated during the generation of plasma. With this arrangement, theparasitic capacity C (pF) between the grounded portion of the housingand hot lines is again less than 1210*f^(−0.9).

Although, in the second embodiment, the thickness of the RF plateelectrode 504 is set to 3 mm, any value may be selected from a rangebetween 1 mm to 6 mm, taking into account the desired stroke and thethickness of the insulator 503 on the RF plate electrode 504. Withinthis range, the parasitic capacity can be reduced efficiently.

<Third Embodiment>

FIG. 9 illustrates a parallel plate plasma processing apparatus 700according to the third embodiment of the invention. The apparatus has aheat sink (or an insulator) 703 for mounting a wafer 701, a DC plate 702positioned near the surface of the heat sink 703, an RF plate electrode704 extending below the DC plate 702, and an opposite electrode 705facing the RF plate electrode. The DC plate 702, the heat sink 703, andthe RF plate electrode 704 comprise a wafer mount electrode.

The plasma processing apparatus 700 also has a DC power source 711 forapplying a DC voltage to the DC plate 702 to hold a wafer 701 in anelectrostatic manner, and an RF power source 709 for applying a radiofrequency to the RF plate electrode 704. Plasma is produced between theRF plate electrode 704 and the opposite electrode 705 by application ofthe radio frequency.

The feature of the third embodiment is a radio frequency trap (referredto as an RF trap) 715 having an electrical length of ¼ of the wavelengthof the applied radio frequency (e.g., 100 MHz in the third embodiment),which is inserted between the DC plate 702 and the DC power source 711.One end of the RF trap 715 is connected to the DC plate 702, and theother end is connected to a 1000 pF bypass capacitor 718 and the DCpower source 711.

The RF trap 715 having an electrical length of ¼ wavelength of theapplied radio frequency, prevents radio frequency from flowing into theDC power source side from the DC plate, thereby eliminating adverseaffects on the DC power source 711.

For example, if the applied radio frequency is 100 MHz and the velocityis equal to the speed of light (c), the λ/4 is obtained from thefollowing equations.

F*λ=c

λ/4=c/4f=3*10⁸ m/(4*100*10⁶)=0.75 m

In practice, the radio frequency propagates through a conductivematerial, and therefore, the value of λ/4 is shorter than 0.75 m. In thethird embodiment, the physical length of the RF trap 715 is 27 cm. TheRF trap 715 is a silver-plated copper pipe with an outer diameter of 1cm. By using a copper pipe, the radio frequency component can beefficiently trapped without preventing the flow of the direct currentapplied to the DC plate 702.

The above-mentioned physical length of the RF trap 715 is determinedusing an oscilloscope. The oscilloscope is connected to the RF plateelectrode 704, and the length of the pipe is adjusted to maximumamplitude. The physical length of 27 cm is shorter than the theoreticalelectrical length of λ/4 (which is v/4f, where v denotes the propagationvelocity in the conductor). This is because of the parasitic capacitythat exists between the hot lines and the grounded portion of thehousing.

The plasma density was measured in the apparatus using the RF trap 715of the third embodiment, and compared with that produced in theconventional apparatus.

In the experiment, 1500V DC voltage was applied to the DC plate 2, andAr gas was introduced from the gas port 6 into the conventionalapparatus shown in FIG. 1. The pressure in the housing 7 was kept at 10Pa. Then, 1 kW RF power was applied to the RF electrode 4 to produceplasma. The electron density of the plasma was measured by a phaseinterferometer for transmission microwave. The measured density was4*10¹¹ cm⁻³.

The same measurement was conducted in the plasma processing apparatus700 with the RF trap 715 under the same conditions. The measured plasmadensity was 5*10¹¹ cm⁻³. The plasma density increases by 20% in theapparatus 700 of the third embodiment, as compared to conventionalapparatus. The increase in plasma density can solve the problem of theprior art, that is, the loss of RF power. In other words, the ratio ofthe RF energy used for generation of plasma to the entire RF energyapplied to the apparatus has increased, and high-speed andhigh-precession plasma processing can be realized.

Although, in the third embodiment, the RF trap 715 is placed before theDC power source 711 used for electrostatic chuck, another RF trap may bepositioned before another DC power source used other purposes. The RFtrap 715 can be inserted in the conductive line between the RF plateelectrode and the chalk coil of the plasma processing apparatus of thefirst embodiment (FIGS. 3, 5 and 6). In this case, the electrical lengthof the RF trap is again λ/4 of the applied radio frequency.

The electrical length of the RF trap is not limited to λ/4. Theelectrical length may be set to 3λ/4, 5λ/4, . . (2n+1)λ/4, as long asthe RF trap can reflect the radio frequency component at the peak of itsamplitude. By inserting the trap having these electrical lengths, theradio frequency transmitted from the wafer mount electrode is reflectedoff the trap and prevented from flowing into the DC power source.Because the DC power source is protected from damages, a loss of RFpower is reduced. Consequently, high-density plasma is produced in theapparatus. In this case, the physical length of the radio-frequency trapis set at less than (2n+1)/4 wavelength of the applied radio frequencytaking into account the parasitic capacity of the plasma processingapparatus and the inductance of transmission lines. The RF trap may bemade of any good conductors, other than copper. The bypass capacitorconnected in parallel to the DC power source supplies a DC voltage tothe RF trap, and has sufficiently low impedance with respect to theradio frequency. The RF voltage becomes zero at the end of the RF trapconnected to the bypass capacitor, and therefore, no RF voltageadversely affects the CD power source.

As has been described above, the plasma processing apparatus of thepresent invention is configured so that the parasitic capacity betweenthe hot lines through which the radio frequency propagates and thegrounded portion of the housing is less than 1210*f^(−0.9).Consequently, the plasma capacity increases relatively, and high-densityplasma processing can be achieved.

A portion of the housing supports the bottom of the heat sink, and theheat sink itself functions as an electrostatic chuck. In thisarrangement, a shock absorber is inserted between the heat sink and thehousing, thereby preventing breakage or cracks of the heat sink at theboundary between the vacuum and the atmosphere. Thus, a compact and asafe plasma processing apparatus can be realized.

The thickness of the RF plate electrode is greatly reduced, and thepositional relationship between the RF plate electrode and the pusherpins is improved so as to substantially eliminate the capacitivecoupling between the RF plate electrode and the pusher pins.

The RF trap also prevents undesirable waste (or loss) of the RF power,and eliminates adverse influence to the DC power source.

The present invention is not limited to the above-described examples,and there are many possible modifications and substitutions. Forexample, the wafer includes any equivalent objects be processed in theplasma processing apparatus of the present invention, and the heat sinkmay be made of any materials that is a good insulator and a good heatradiator. Plasma processing includes not only plasma etching, but alsofilm formation, such as CVD. The plasma processing apparatus of thepresent invention can also be applied to plasma ion implantation.

What is claimed is:
 1. A plasma processing apparatus comprising: agrounded housing; an RF plate electrode placed in the housing; anopposite electrode facing the RF plate electrode; and an RF power sourcefor applying a radio frequency to either the RF plate electrode or theopposite electrode to generate plasma between the two electrodes,wherein the radio frequency is 60 MHz or higher, and when the radiofrequency is represented as f (MHz), a parasitic capacity C (pF) betweenthe grounded portion of the housing and a conductive portion throughwhich the radio frequency propagates is less than 1210*f^(−O.9).
 2. Theplasma processing apparatus of claim 1, wherein the RE late electrodehas a thickness of 6 mm or less.
 3. The plasma processing apparatus ofclaim 1, further comprising a heat sink that holds the RF plateelectrode and has a coolant passage inside it.
 4. The plasma processingapparatus of claim 3, wherein the heat sink further has a groove insideit.
 5. The plasma processing apparatus of claim 3, wherein the heat sinkis supported by a part of the housing, and a shock absorber is insertedbetween the heat sink and said part of the housing.
 6. The plasmaprocessing apparatus of claim 5, wherein the heat sink is made of aceramic material and the shock absorber is a malleable insulator.
 7. Theplasma processing apparatus of claim 5, wherein the shock absorber ismade of Teflon.
 8. The plasma processing apparatus of claim 1, furthercomprising: a DC power source for supplying a direct voltage in order tohold a wafer on the RF plate electrode in an electrostatic manner; and aradio-frequency trap positioned between the DC power source and the RFplate electrode, the radio-frequency trap having an electrical length of1/4 wavelength of the radio frequency applied to the electrode.
 9. Aplasma processing apparatus comprising: a housing having a shoulder; aheat sink configured such that a contact area between a bottom of theheat sink and the shoulder is smaller than an area of the bottom of theheat sink; an RF plate electrode placed on a top of the heat sink in thehousing; an opposite electrode facing the RF plate electrode; and an RFpower source applying a radio frequency to either the RF plate electrodeor the opposite electrode to generate plasma between the two electrodes,wherein the radio frequency is 60 MHz or higher, and when the radiofrequency is represented as f (MHz), a parasitic capacity (pF) between agrounded portion of the housing and a conductive portion through whichthe radio frequency propagates is less than 1210*f^(−0.9).
 10. Theplasma processing apparatus of claim 9, wherein the RF plate electrodeas a thickness of 6 mm or less.
 11. The plasma processing apparatus ofclaim 9, wherein the heat sink holds the plate electrode and has acoolant passage inside it.
 12. The plasma processing apparatus of claim11, wherein the heat sink further as a groove inside it.
 13. The plasmaprocessing apparatus of claim 9, wherein a shock absorber is insertedbetween the heat sink and said shoulder of the housing.
 14. The plasmaprocessing apparatus of claim 13, wherein the heat sink is made of aceramic material and the shock absorber is a malleable insulator. 15.The plasma processing apparatus of claim 13, wherein the shock absorberis made of Teflon.
 16. The plasma processing apparatus of claim 9,further comprising: a DC power source supplying a direct voltage inorder to hold a wafer on the RF plate electrode in an electrostaticmanner; and a radio-frequency trap positioned between the DC powersource and the RF plate electrode, the radio-frequency trap having anelectrical length of 1/4 wavelength of the radio frequency applied tothe electrode.